Seismic Hazard Model Research Articles

Best Matching Scenario Earthquake: an alternative paradigm to disaggregation of PSHA

Disaggregation of Probabilistic Seismic Hazard Assessment (PSHA) is commonly employed to estimate the most likely magnitude and distance values (Mw - R) contributing to the hazard at the site of study. Nevertheless, however defined, probabilistic disaggregation techniques are affected by several ambiguities. Among the others, they require the selection of a specific branch of the logic tree used to perform the PSHA, and of a specific structural period, which may be of limited use in some engineering applications, such as for structures with strong participation of higher modes and highly affected by non-linear response, as well as site response analyses and most of the geotechnical engineering applications. In order to overcome some of these drawbacks, this paper introduces a new possibility aiming at estimating a Best Matching Scenario Earthquake (BMSE) defined as the earthquake, specified in terms of (Mw - R), the median spectral ordinates of which best approach a target uniform hazard spectrum (UHS) for the range of structural periods of interest. A more general form of the BMSE can be introduced, i.e., in terms of (Mw – R –) instead of (Mw – R – =0), but the paper shows that excellent results can be obtained in most cases along the Italian territory, by limiting the search to =0. In this article, after highlighting the assumptions behind the method, a few examples of the application of the BMSE with reference to the current seismic hazard model of Italy are shown. Our results show the potential effectiveness of the best matching method that gives in most cases scenarios which are consistent with historical observations. Besides normative implications, such as the selection of input motions for non-linear time history analyses, the BMSE may provide a rationale support for using seismic ground shaking scenarios in a probabilistic framework.

A database of ground motion recordings, site profiles, and amplification factors from the Groningen gas field in the Netherlands

A comprehensive database that has been used to develop ground motion models for induced earthquakes in the Groningen gas field is provided in a freely accessible online repository. The database includes more than 8500 processed ground motion recordings from 87 earthquakes of local magnitude ML between 1.8 and 3.6, obtained from a large network of surface accelerographs and borehole geophones placed at 50 m depth intervals to a depth of 200 m. The 5%-damped pseudo-acceleration spectra and Fourier amplitude spectra of the records are also provided. Measured shear-wave velocity (VS) profiles, obtained primarily from seismic Cone Penetration Tests (CPTs), are provided for 80 of the ∼100 recording stations. A model representing the regional dynamic soil properties is presented for the entire gas field plus a 5 km onshore buffer zone, specifying lithology, VS, and damping for all layers above the reference baserock horizon located at about 800 m depth. Transfer functions and frequency-dependent amplification factors from the reference rock horizon to the surface for the locations of the recording stations are also included. The database provides a valuable resource for further refinement of induced seismic hazard and risk modeling in Groningen as well as for generic research in site response of thick, soft soil deposits and the characteristics of ground motions from small-magnitude, shallow-focus induced earthquakes.

Estimation of Damage Scenarios in Mexico City Caused by Nearby Crustal Earthquakes

ABSTRACT Mexico City has sustained severe damage from earthquakes located in the subduction zone and within the subducted Cocos plate. The city lies in the trans-Mexican volcanic belt (TMVB). Earthquakes in the TMVB are relatively infrequent but have reached Mw>6. A recent Mw 6.9 crustal earthquake occurred in 1912, 80 km from Mexico City. Damage scenarios are estimated for two hypothetical crustal earthquakes located 40 and 80 km from the city. The seismic sources represent hazard-critical earthquakes and are based on historical seismicity and on the geological record. The probability distribution of ground acceleration is determined by ground-motion models (GMMs) from similar tectonic regions. Site effects are included to account for the geotechnical zonation of the city, and vulnerability curves are calculated for residential buildings in the cadastral record of Mexico City. The results show that an earthquake near the 1912 earthquake would produce accelerations of 37 and 148 cm/s2 on firm ground and in the lake region, respectively. In this scenario, damage is concentrated in buildings 1-2 stories high located near the transition zone. The earthquake located ∼40 km from Mexico City shows accelerations of 79 and 318 cm/s2 on firm ground and in the lake region, respectively. This scenario concentrated potential damage in buildings 1-2 stories high and broadly distributed in the region underlain by soft clays. Under both scenarios, no damage is expected in buildings higher than ∼11 stories. A deterministic approach using two potential seismic sources instead of a probabilistic seismic hazard model is used because of the lack of background seismicity and the unknown recurrence times of events in the TMVB. The results show very different damage patterns for crustal earthquakes. The two scenarios presented here may be useful for decision makers and insurance companies to estimate physical damage and to implement mitigation programs.

Seismic fatality risk evaluation framework for existing buildings in Switzerland

The Swiss code SIA 269/8 for the seismic evaluation of existing structures is risk-based. It relies on the compliance factor αeff, defined as the ratio of the seismic capacity of an existing structure to the required seismic capacity of a hypothetical identical new structure, to indirectly evaluate the casualty risk posed by the existing structure. While it is convenient to have a direct relationship between the casualty risk and the degree of code compliance of the existing structure, it is debatable if the contributions of the seismic hazard, structural performance and occupancy to the casualty risk should be merged into a single quantity, conflicting with the multi-component nature of seismic risk. This paper presents a graphical framework to evaluate the seismic fatality risk for existing buildings. The framework is based on the PEER Center performance-based earthquake engineering methodology to provide a transparent view of the contributions of different sources of hazard and risk. Starting from a seismic hazard model, probabilistic seismic demand, damage and loss models are introduced, taking the uncertainty into account at each step. The final product of the proposed framework, a loss curve representing the (annual) exceedance rate of fatality counts, enables engineers and building owners to make better, risk-based decisions. The proposed framework is demonstrated for an existing building in Switzerland. Notably, in addition to the SIA 269/8 evaluation method, this demonstration compares eight hybrid approaches differentiated by the utilized structural assessment techniques (nonlinear static pushover vs. incremental dynamic analysis), uncertainty propagation in the demand model (record variability only vs. record variability and epistemic uncertainty) and solution strategies (closed-form vs. numerical) to find a practical compromise between the accuracy and the computational effort. A comparison of the obtained results shows that the SIA 269/8 evaluation method underestimates the fatality risk for the presented building and identifies a practical alternative based on the proposed graphical framework.

Modelling site response at regional scale for the 2020 European Seismic Risk Model (ESRM20)

Quantitative estimation of seismic risk over a region requires both an underlying probabilistic seismic hazard model and a means to characterise shallow site response over a large scale. The 2020 European Seismic Risk Model (ESRM20) builds on the 2020 European Seismic Hazard Model (ESHM20), requiring additional information to firstly parameterise the local site condition across all of Europe, and subsequently determine its influence on the prediction of seismic ground motion. Initially, a harmonised digital geological database for Europe is compiled, alongside a model of topographic/bathymetric elevation and a database of 30 m averaged shearwave velocity measurements (V_{S30}), in order to produce separate 30 arc-second maps of inferred V_{S30} based on topography and on geology. We then capitalise on a large database of seismic recording stations in Europe for which site-to-site ground motion residuals (delta S2S_{S}) have been determined with respect to the shallow crustal ground motion model used in the ESHM20. These residuals allow us to incorporate site amplification functions into the European GMM calibrated upon either observed or inferred V_{S30}, or on the European geology and topography models. We present the resulting pan-European seismic site amplification model and assess its impact on seismic hazard and risk compared against other approaches. The new site amplification model fulfils the requirements of the ESRM20 and, providing uncertainty is fully propagated, yields estimates of seismic hazard and risk at a large space scale that may be comparable to other methods often applied at local/urban scale where better-constrained site information is available.

Probabilistic seismic source inversion of the 1886 Charleston, South Carolina, earthquake from macroseismic evidence: A major updating

The source of the 1886 Charleston, South Carolina earthquake influences the computed seismic hazard of the Southeastern U.S. and thus impacts public policy and engineering practice. However, because the 1886 rupture predated seismic instruments, its source is highly uncertain. This study presents probabilistic seismic-source inversions of the Charleston earthquake from liquefaction evidence and historical intensity reports. Using the latest predictive models and a novel inversion approach, we seek to constrain the magnitude, location, and orientation of the 1886 rupture. Probability distributions of rupture magnitude are conditioned on both the “Woodstock Fault” – a commonly inferred source of the 1886 event – and on an unknown source, wherein the uncertainties of fault location and orientation are considered. These distributions are compared to the Mw6.7-Mw7.5 distribution adopted by the U.S. National Seismic Hazard Model Project (NSHMP). Collectively, the results do not provide strong support for the hypothesized Woodstock Fault. This is not to say the Woodstock Fault does not exist or could not have caused the 1886 rupture, but rather, that the position of the 1886 source model cannot be constrained by the data and models studied herein, given the large uncertainties inherent to each. While this is at odds with the underlying assumption of prior studies, the results nonetheless generally uphold the magnitude distribution adopted by the NSHMP. The largest uncertainties inherent to this distribution are identified and could be diminished in the future. Finally, we note that the inversion methodology used here is not specific to any region, or to certain types of evidence, but can be applied to any seismic zone and to any co-seismic response. This methodology allows for uncertainty to be accounted for in a more complete and transparent manner when inverting seismic source parameters from macroseismic data. Of course, any limitations, biases, or unmodeled uncertainties inherent to these data must be understood, and their implications acknowledged, as further discussed herein.

The European Preinstrumental Earthquake Catalogue EPICA, the 1000–1899 catalogue for the European Seismic Hazard Model 2020

Abstract. The European PreInstrumental Earthquake CAtalogue (EPICA) (Rovida and Antonucci, 2021; https://doi.org/10.13127/epica.1.1) is the 1000–1899 seismic catalogue compiled for the European Seismic Hazard Model 2020 (ESHM20), an outcome of the project Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe (SERA), in the framework of the European Union's Horizon 2020 research and innovation programme. EPICA is the update of the SHARE European Earthquake Catalogue (SHEEC) 1000–1899, with which it shares the main principles – mostly transparency, repeatability and continent-wide harmonisation of data – as well as the compilation strategies and methods. Version 1.1 of EPICA contains 5703 earthquakes with either maximum intensity ≥5 or Mw≥4.0, with a spatial coverage from the Atlantic Ocean to the west to 32∘ E in longitude, and from the Mediterranean Sea to Northern Europe. EPICA relies upon the updated knowledge of the European preinstrumental seismicity provided by the data gathered in the European Archive of Historical Earthquake Data (AHEAD). Such data are both macroseismic intensity data supplied by descriptive historical seismological studies and online macroseismic databases, and parameters contained in regional catalogues. As done for the compilation of SHEEC 1000–1899, these datasets were thoroughly analysed in order to select the most representative of the knowledge of each earthquake, independently from national constraints. Selected intensity distributions are processed with three methods to determine location and magnitude based on the attenuation of macroseismic intensity and are combined with parameters harmonised from modern regional catalogues. This paper describes the compilation procedure of EPICA version 1.1, its input data, the assessment of the earthquake parameters and the resulting catalogue, which is finally compared with its previous version. Technical solutions for accessing the catalogue, both as downloadable files and through web services, are also illustrated.

A Caribbean and Central America Seismic Hazard Model for Sovereign Parametric Insurance Coverage

ABSTRACTA fully probabilistic seismic hazard model with a single domain and sufficiently accurate resolution level for national analyses has been developed, for the Caribbean and Central America, to support the design of parametric earthquake policies offered by the Caribbean Catastrophe Risk Financing Facility to sovereign countries. This model provides updated earthquake hazard and risk information for 34 countries in the region, allowing to obtain detailed seismic hazard results at any location within the area of analysis. Besides a detailed zonation of subduction and crustal sources, updated seismicity and tectonic data have been utilized. Different seismicity, geometric, and rupture models have been adopted using state-of-the-art methodologies and tools. The main output of this model is a stochastic event-set, which is the hazard representation utilized not only to compute a long-term overview of the seismic hazard in the region but also to probabilistically estimate earthquake modeled losses at a national level that are used as triggers for underpinning parametric earthquake insurance. Although not of direct interest for parametric insurance purposes, this model also yields hazard maps and uniform hazard spectra for different return periods. For the operational purposes of the parametric insurance coverage, a postevent tool was developed to calculate, in near-real time, the ground-motion intensities associated with any earthquake within the region under study. This is done automatically by choosing a rupture plane from the moment tensor solution based on predefined rules while maintaining full consistency with all the assumptions made in the probabilistic seismic hazard analysis. The predicted ground motions in the region and the vulnerability functions developed for the exposed assets are then used to estimate whether the economic losses for each affected country exceed the trigger value stated in the country-specific insurance policy conditions and whether a payout should be issued, which is typically disbursed within 14 days.

Seismic collapse risk of RC-timber hybrid building with different energy dissipation connections considering NBCC 2020 hazard

The 2020 National Building Code of Canada (NBCC) seismic hazard model (SHM) marks a comprehensive update over its predecessor (NBCC 2015). For different regions in Canada, this will have an impact on the design of new buildings and performance assessment of existing ones. In the present study, a recently developed hybrid building system with reinforced concrete (RC) moment-resisting frames and cross-laminated timber (CLT) infills is assessed for its seismic performance against the latest SHM. The six-story RC-CLT hybrid system, designed using the direct displacement-based method, is located in Vancouver, Canada. Along with very high seismicity, southwestern British Columbia is characterized by complex seismotectonics, consisting of subduction, shallow crustal, and in-slab faulting mechanisms. A hazard-consistent set of 40 ground motion pairs is selected from the PEER and KiK-net databases, and used to estimate the building’s seismic performance. The effects of using steel slit dampers (associated with large hysteresis loops) and flag-shaped energy dissipators (associated with the recentering capability) are investigated. The results indicate that the hybrid system has good seismic performance with a probability of collapse of 2–3% at the 2475-year return period shaking intensity. The hybrid building with steel slit dampers exhibits a collapse margin ratio of 2.8, which increases to 3.5–3.6 when flag-shaped dissipators are used. The flag-shaped dissipators are found to significantly reduce the residual drift of the hybrid building. Additionally, the seismic performance of the hybrid building equipped with flag-shaped dissipators is found to improve marginally when the recentering ratio is increased.

Revealing Crustal Deformation and Strain Rate in Taiwan Using InSAR and GNSS

AbstractInterseismic deformation describes the gradual accumulation of crustal strain within the tectonic plate and along the plate boundaries before the sudden release as earthquakes. In this study, we use 5 years of high spatial and temporal geodetic measurements, including Global Navigation Satellite System and Interferometric Synthetic Aperture Radar to monitor 3‐dimension interseismic crustal deformation and horizontal strain rate in Taiwan. We find significant deformation (strain rate >8 10−6 yr−1) along the plate boundary between the Philippine Sea and the Eurasian Plates in east Taiwan. The high strain rate in the southern part of the Western Foothills is distributed along a few major fault systems, which reveals the geometry of the deformation front in west Taiwan. Our results help identify active faults in southwest and north Taiwan that were not identified before. These findings can be insightful in informing future seismic hazard models.

Variations in uniform hazard spectra and disaggregated scenarios during earthquake sequences

Seismic hazard varies greatly during an earthquake sequence. Understanding this variation can be useful to end-users, such as emergency managers, as it would enable them to make more informed decisions about potential risk reduction measures. This article presents examples of how two commonly-used products of probabilistic seismic hazard assessments: uniform hazard spectra and disaggregated earthquake scenarios, vary during two severe seismic sequences in western Greece. These calculations are made using a recent time-dependent seismic hazard model based on a Bayesian ETAS approach. The examples show that time-dependent uniform hazard spectra for short return periods (1 and 10 years) are significantly higher than standard time-independent spectra but that uniform hazard spectra for the commonly-used return periods of 475 and 2475 years are similar to those from time-independent assessments. The time-dependent spectra generally converge within a couple of days to the time-independent spectra. The examples also show that the dominant earthquake scenarios evidenced by the disaggregation for the time-dependent assessment can show significant differences from the time-independent scenarios. This is particularly true when the earthquake sequence is distant from the location of interest as the aftershocks contribute greatly to the overall hazard. To show these changes more clearly this article introduces a new graphical representation of the disaggregated results: contour maps showing the magnitude or distance of the dominant earthquake scenario with axes of the structural period and response spectral acceleration.

A probabilistic seismic hazard model for Greenland

Greenland belongs to a stable continental region where seismic activity is low. Due to this factor and low population, only one probabilistic seismic hazard model of Greenland has been published, showing only a peak ground acceleration map. We constructed a probabilistic seismic hazard model for Greenland in order to complete our global earthquake risk map. We used a smoothed seismicity method due to the lack of recognized active faults. An earthquake magnitude-frequency distribution was derived based on the recorded earthquakes from 1933 to 2013. We assumed a maximum earthquake magnitude of 7.0 for the distribution, given that the magnitude of the largest recorded tectonic earthquake in the region is about 6.0. The NGA-East ground motion models were employed for ground shaking calculations. Finally, we presented seismic hazard maps in terms of peak ground acceleration, spectral acceleration at 0.2 ​s and 1.0 ​s, and hazard curves for several of the major cities.

Revised Earthquake Geology Inputs for the Central and Eastern United States and Southeast Canada for the 2023 National Seismic Hazard Model

AbstractIt has been about a decade since updates to seismic and fault sources in the central and eastern United States (CEUS) were last assessed for the 2012 Central and Eastern United States Seismic Source Characterization for nuclear facilities (CEUS-SSCn) and 2014 U.S. Geological Survey National Seismic Hazard Model (NSHM) for the conterminous United States. In advance of the 2023 NSHM update, we created three related geospatial databases to summarize and characterize new fault source information for the CEUS. These include fault section, fault-zone polygon, and earthquake geology (fault slip rate, earthquake recurrence intervals) databases that document updates to fault parameters used in the prior seismic hazard models in this region. The 2012 CEUS-SSCn and 2014 NSHM fault models served as a foundation, as we revised and added fault sources where new published studies documented significant changes to our understanding of fault location, geometry, or activity. We added nine new fault sections that meet the criteria of (1) a length ≥7 km, (2) evidence of recurrent Quaternary tectonic activity, and (3) documentation that is publicly available in a peer-reviewed source. The prior CEUS models only included six fault sections (sources) and 10 fault-zone polygons (previously called repeating large-magnitude earthquake [RLME] polygons). The revised databases include 15 fault sections and 12 fault-zone polygons. Updates to the faults constitute a 150% increase in fault sections, and two additional fault-zone polygons, although some fault-zone polygons differ from RLME polygons used in the prior models. No faults were removed from the past models. Several seismic zones and suspected faults were evaluated but not included in this update due to a lack of information about fault location, geometry, or recurrent Quaternary activity. These updates to the fault sections, fault-zone polygons, and earthquake geology databases will inform fault geometry and activity rates of CEUS sources during the 2023 NSHM implementation.

Lower Seismogenic Depth Model for Western U.S. Earthquakes

Abstract We present a model of the lower seismogenic depth of earthquakes in the western United States (WUS) estimated using the hypocentral depths of events M > 1, a crustal temperature model, and historical earthquake rupture depth models. Locations of earthquakes are from the Advanced National Seismic System Comprehensive Earthquake Catalog from 1980 to 2021 supplemented with seismicity in southern California for event hypocenters that were relocated by Hauksson et al. (2012) to obtain higher precision and better resolution in the model. We calculated the average depth of the deepest 10% of the merged catalog using an adaptive radius of 50 km or more. Along the San Andreas fault, the deepest seismogenic depths are located at 23 km around the Cholame segment, whereas the shallowest depths are located at about 10 km along the Rodgers Creek and Maacama faults. For the WUS outside California, the depth generally varies between 10 and 25 km with an average around 14 km but could extend to 35 km along Cascadia subduction zone. We find good agreement between the small-magnitude depths and rupture depths derived from coseismic slip of large earthquakes across the region. Our estimates are generally deeper than the previous seismogenic depths determined for the Uniform California Earthquake Rupture Forecast, Version 3 model based on work by Petersen et al. (1996) who used seismicity cross sections along major fault zones in California. Our new seismogenic depth distribution correlates closely with crustal temperature derived from WUS heat flow (Blackwell et al., 2011). This correlation allowed us to develop a map of the brittle–ductile transition that we use to replace seismogenic depths in the model east of the Intermountain West Seismic Belt where the seismicity rate is low. This updated depth model is useful for recalibrating the lower geologic fault rupture depths, and constraining deformation and seismicity source models in updates of the U.S. Geological Survey National Seismic Hazard Model.

A Fault-Based Crustal Deformation Model with Deep Driven Dislocation Sources for the 2023 Update to the U.S. National Seismic Hazard Model

Abstract A fault-based crustal deformation model with deep driven dislocation sources is applied to estimate long-term on-fault slip rates and off-fault moment rate distribution in the western United States (WUS) for the 2023 update to the National Seismic Hazard Model (NSHM). This model uses the method of Zeng and Shen (2017) to invert for slip rate and strain-rate parameters based on inputs from Global Positioning System (GPS) velocities and geologic slip-rate constraints. The model connects adjacent major fault segments in California and the Cascadia subduction zone to form blocks that extend to the boundaries of the study area. Faults within the blocks are obtained from the NSHM geologic fault section database. The model slip rates are determined using a least-squares inversion with a normalized chi-square of 6.6. I also apply a time-dependent correction called “ghost transient” effect to account for the viscoelastic responses from large historic earthquakes along the San Andreas fault and Cascadia subduction zone. Major discrepancies between model slip rates and geologic slip rates along the San Andreas fault, for example, from the Cholame to the Mojave and San Bernardino segments of the San Andreas, are well reduced after the ghost transient correction is applied to GPS velocities. The off-fault moment rate distribution is consistent with regional tectonics and seismicity patterns with a total rate of 1.6×1019 N·m/yr for the WUS.

Preface to the Focus Section on Deformation Models for the U.S. National Seismic Hazard Model

Preface to the Focus Section on Deformation Models for the U.S. National Seismic Hazard Model

Creep Rate Models for the 2023 US National Seismic Hazard Model: Physically Constrained Inversions for the Distribution of Creep on California Faults

Abstract Widespread surface creep is observed across a number of active faults included in the United States (US) National Seismic Hazard Model (NSHM). In northern California, creep occurs on the central section of the San Andreas fault, along the Hayward and Calaveras faults through the San Francisco Bay Area, and to the north coast region along the Maacama and Bartlett Springs faults. In southern California, creep is observed across the Coachella segment of the San Andreas fault, through the Brawley Seismic Zone, and along the Imperial and Superstition Hills faults. Seismic hazard assessments for California have accounted for creep using various data and methods, including the most recent Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) in 2013. The purpose of this study is to expand and update the UCERF3 creep rate data set for the 2023 release of the US NSHM and to invert geodetic data and the surface creep rate data for the spatial distribution of interseismic fault creep on California faults using an elastic model with physical creep constraints. The updated surface creep rate compilation consists of a variety of data types including alignment arrays, offset cultural markers, creepmeters, Interferometric Synthetic Aperture Radar, and Global Positioning System data. We compile a total of 497 surface creep rate measurements, 400 of which are new and 97 of which appear in the UCERF3 compilation. We compute creep rate distributions for each of the five 2023 NSHM geodetic-based and geologic-based deformation models. Computed creep rates are used to reduce the total fault moment rate available for earthquake sequences in the NSHM model. We find that, despite relatively large variability in model long-term slip rates across all five deformation models, the variability in depth-averaged creep rate across all models is relatively small, typically 5–10 mm/yr along the creeping San Andreas fault section and only 2–4 mm/yr along the Maacama and Rodgers Creek-Hayward faults.

GPS Velocity Field of the Western United States for the 2023 National Seismic Hazard Model Update

Abstract Global Positioning System (GPS) velocity solutions of the western United States (WUS) are compiled from several sources of field networks and data processing centers for the 2023 U.S. Geological Survey National Seismic Hazard Model (NSHM). These solutions include both survey and continuous-mode GPS velocity measurements. I follow the data processing procedure of Parsons et al. (2013) for the Uniform California Earthquake Rupture Forecast, version 3 and McCaffrey, Bird, et al. (2013) and Zeng and Shen (2013) for their WUS deformation models in support of the 2014 NSHM update. All GPS velocity vectors are first rotated to a common North American reference frame. I edit the velocities to remove outliers and data with significant influence from volcanism. The solutions are then combined into a final GPS velocity field consisting of 4979 horizontal velocity vectors. I compute strain rates based on these GPS velocities using the method of Shen et al. (2015). These strain rates correlate closely with seismicity rates in the WUS. The results are used for WUS geodetic and geologic deformation modeling in support of the 2023 NSHM update.

Western U.S. Deformation Models for the 2023 Update to the U.S. National Seismic Hazard Model

Abstract This report describes geodetic and geologic information used to constrain deformation models of the 2023 update to the National Seismic Hazard Model (NSHM), a set of deformation models to interpret these data, and their implications for earthquake rates in the western United States. Recent updates provide a much larger data set of Global Positioning System crustal velocities than used in the 2014 NSHM, as well as hundreds of new faults considered as active sources for the 2023 NSHM. These data are interpreted by four geodetic models of deformation that estimate fault slip rates and their uncertainties together with off-fault moment release rates. Key innovations in the 2023 NSHM relative to past practice include (1) the addition of two new (in addition to two existing) deformation models, (2) the revision and expansion of the geologic slip rate database, (3) accounting for fault creep through development of a creep-rate model that is employed by the four deformation models, and (4) accounting for time-dependent earthquake-cycle effects through development of viscoelastic models of the earthquake cycle along the San Andreas fault and the Cascadia subduction zone. The effort includes development of a geologic deformation model that complements the four geodetic models. The current deformation models provide a new assessment of outstanding discrepancies between geologic and geodetic slip rates, at the same time highlighting the need for both geologic and geodetic slip rates to robustly inform the earthquake rate model.

Western U.S. Geologic Deformation Model for Use in the U.S. National Seismic Hazard Model 2023

Abstract Fault geometry and slip rates are key input data for geologic deformation models, which are a fundamental component of probabilistic seismic hazard analyses (PSHAs). However, geologic sources for PSHA have traditionally been limited to faults with field-based slip rate constraints, which results in underrepresentation of known, but partially characterized, active faults. Here, we evaluate fault geometries and geologic fault slip rates for the western United States to construct a new geologic deformation model for the U.S. National Seismic Hazard Model 2023 update (NSHM23). In previous NSHM iterations, only faults with published geologic slip rates were included. In the NSHM23 fault sections database compilation, this inclusion criterion was expanded to include faults without known slip rates. In this updated geologic deformation model, preferred slip rates and associated uncertainty distributions are incorporated for faults with slip rates derived from field studies. For faults without site-specific slip rates, we evaluate a suite of uncertainty distributions derived from broad slip rate categories in the U.S. Geological Survey Quaternary Fault and Fold Database. Preferred slip rate distributions are selected via comparison with geodetic strain rates in tectonic subregions. The resultant moment of the geologic deformation model is generally in deficit compared with the geodetic moment within each region. Primary advances in the NSHM23 geologic deformation model include the following: (1) slip rates are presented as preferred values with uncertainties rather than single values; (2) the representation of the western U.S. active fault network is more complete; and (3) the geologic deformation model leverages geodetic information to assess regional constraints on geologic fault slip rates.